He said: "I think what happens when you get into your third term and you are coming up to your tenth year is that it really doesn't matter what you say, you are going to get people saying it should be time for a change or when are you going or who's taking over?

"You just get on with the job because this speculation, I think, would happen whatever decision you take.

"Now, it was an unusual thing for me to say but people kept asking me the question so I decided to answer it. Maybe that was a mistake."

'Date in mind'?

Mr Blair has insisted he will serve a full third term in office before resigning. But there has been constant speculation about when he will quit and hand over to Chancellor Gordon Brown.

Newspapers on Monday reported that the prime minister was understood to have a firm date in mind for his departure, but even his closest aides did not know when it is.

It is unlikely to be as soon as some Labour rebels are hoping.

Asked about Mr Blair's comments, Deputy Prime Minister John Prescott told the BBC: "He might mean it caused an awful lot of uncertainties once he said that, and that was a concern expressed by a number at the time."

But Mr Prescott said he did not think Mr Blair's announcement had been a mistake because it enabled a smooth transfer of power.

Asked if the Australian interview would speed up that process, Mr Prescott added: "I still think that the timetable in people's minds is still reasonably the same."

'Exit strategy needed'

A spokeswoman for Number 10 said the prime minister had been cut off mid-sentence on the radio when the interviewer talked over his comments.

She said he intended to say: "It was a mistake... to believe that the announcement would kill off the speculation as to when I would resign."

If the going is tough, we tough it out

Tony Blair

Lord Falconer, the Lord Chancellor, echoed that opinion, adding: "What is said is that it was a mistake to think it would stop the speculation."

But Conservative leader David Cameron said Mr Blair's comments had given rise to "a lot of uncertainty".

"Every organisation needs a succession plan and the government of Britain is no different," he said.

Liberal Democrat leader Sir Menzies Campbell also urged Mr Blair "to tell us his own exit strategy", saying his recent remarks added to the "current uncertainty and sense of drift".

"The issue now is not the interests of the Labour party but the good governance of Britain," he said.

'Take courage'

The prime minister turned his attention to global matters in a speech to the Australian Parliament in Canberra.

He said: "If we want to secure our way of life, there's no alternative but to fight for it.

"That means standing up for our values not just in our own country but the world over."

On the situation in Iraq he said: "If the going is tough, we tough it out. This is not a time to walk away but to have the courage to see it through."